It is known in the art relating to internal combustion engines, including two-cycle diesel engines, to provide pistons reciprocable in the engine cylinders and sealed by compression rings near the closed (combustion chamber) end of the cylinder. One or more oil control rings are commonly provided for scraping excess lubricating oil from the cylinder walls.
In ported cylinder engines, such as for example some two-cycle diesel engines having intake ports intermediate the ends of the cylinders, the oil control rings are conventionally placed near the bottom edge of their piston skirt. They are thus located so that these rings do not travel above the lower edges of the air inlet ports during reciprocation of the piston in its cylinder. Thus, the oil control rings in ported cylinder engines generally have the dual purposes of (1) preventing the loss of excessive lubricating oil into the engine inlet air chamber and the combustion chamber and (2) sealing the cylinder against the excessive loss of intake air from the air chamber into the engine crankcase.
In the past it has been common in combustion engine practice with both ported cylinder engines and other engines, to provide some type of drain passage or passages extending through the piston walls either at, or immediately below the oil ring grooves. These passages are provided to drain to the interior of the piston, and thus to the engine crankcase, oil scraped by the oil rings from the cylinder walls, so as to prevent the excessive buildup of oil at the scraping edges of the rings. However, this arrangement of oil control features, particularly when applied to oil cooled pistons in heavy duty engines such as for example two-cycle diesel engines, has sometimes permitted flooding of the oil control rings with cooling oil from the interior of the piston by passage through the drain openings. Also, such openings have permitted the free flow to the crankcase of scavenging air which leaks past the top oil control ring, thus by-passing the second oil control ring completely.